Merge pull request #195 from tpdickso/ungenerate-nans

Add feature flag to disable floating point exceptions in unexceptional code

build/rapier2d-f64/Cargo.toml

@@ -20,6 +20,7 @@ default = [ "dim2", "f64", "default-sets" ]
 dim2    = [ ]
 f64     = [ ]
 default-sets = [ ]
+avoid-fe-exceptions = [ ]
 parallel = [ "rayon" ]
 simd-stable = [ "simba/wide", "simd-is-enabled" ]
 simd-nightly = [ "simba/packed_simd", "simd-is-enabled" ]

build/rapier2d/Cargo.toml

@@ -20,6 +20,7 @@ default = [ "dim2", "f32", "default-sets" ]
 dim2    = [ ]
 f32     = [ ]
 default-sets = [ ]
+avoid-fe-exceptions = [ ]
 parallel = [ "rayon" ]
 simd-stable = [ "simba/wide", "simd-is-enabled" ]
 simd-nightly = [ "simba/packed_simd", "simd-is-enabled" ]

build/rapier3d-f64/Cargo.toml

@@ -20,6 +20,7 @@ default = [ "dim3", "f64", "default-sets" ]
 dim3    = [ ]
 f64     = [ ]
 default-sets = [ ]
+avoid-fe-exceptions = [ ]
 parallel = [ "rayon" ]
 simd-stable = [ "parry3d-f64/simd-stable", "simba/wide", "simd-is-enabled" ]
 simd-nightly = [ "parry3d-f64/simd-nightly", "simba/packed_simd", "simd-is-enabled" ]

build/rapier3d/Cargo.toml

@@ -20,6 +20,7 @@ default = [ "dim3", "f32", "default-sets" ]
 dim3    = [ ]
 f32     = [ ]
 default-sets = [ ]
+avoid-fe-exceptions = [ ]
 parallel = [ "rayon" ]
 simd-stable = [ "parry3d/simd-stable", "simba/wide", "simd-is-enabled" ]
 simd-nightly = [ "parry3d/simd-nightly", "simba/packed_simd", "simd-is-enabled" ]

src/dynamics/solver/velocity_constraint.rs

@@ -385,7 +385,13 @@ where
     let relative_linvel = linvel1 - linvel2;
     let mut tangent_relative_linvel =
         relative_linvel - force_dir1 * (force_dir1.dot(&relative_linvel));
-    let tangent_linvel_norm = tangent_relative_linvel.normalize_mut();
+
+    let tangent_linvel_norm = {
+        let _disable_fe_except =
+            crate::utils::DisableFloatingPointExceptionsFlags::disable_floating_point_exceptions();
+        tangent_relative_linvel.normalize_mut()
+    };
+
     let threshold: N::Element = na::convert(1.0e-4);
     let use_fallback = tangent_linvel_norm.simd_lt(N::splat(threshold));
     let tangent_fallback = force_dir1.orthonormal_vector();

src/dynamics/solver/velocity_constraint_element.rs

@@ -99,7 +99,13 @@ impl<N: SimdRealField> VelocityConstraintTangentPart<N> {
                 self.impulse[0] - self.r[0] * dimpulse_0,
                 self.impulse[1] - self.r[1] * dimpulse_1,
             );
-            let new_impulse = new_impulse.simd_cap_magnitude(limit);
+            let new_impulse = {
+                let _disable_fe_except =
+                    crate::utils::DisableFloatingPointExceptionsFlags::
+                    disable_floating_point_exceptions();
+                new_impulse.simd_cap_magnitude(limit)
+            };
+
             let dlambda = new_impulse - self.impulse;
             self.impulse = new_impulse;
 

src/dynamics/solver/velocity_ground_constraint_element.rs

@@ -78,7 +78,12 @@ impl<N: SimdRealField> VelocityGroundConstraintTangentPart<N> {
                 self.impulse[0] - self.r[0] * dimpulse_0,
                 self.impulse[1] - self.r[1] * dimpulse_1,
             );
-            let new_impulse = new_impulse.simd_cap_magnitude(limit);
+            let new_impulse = {
+                let _disable_fe_except =
+                    crate::utils::DisableFloatingPointExceptionsFlags::
+                    disable_floating_point_exceptions();
+                new_impulse.simd_cap_magnitude(limit)
+            };
             let dlambda = new_impulse - self.impulse;
 
             self.impulse = new_impulse;

src/utils.rs

@@ -670,6 +670,52 @@ impl Drop for FlushToZeroDenormalsAreZeroFlags {
     }
 }
 
+// This is an RAII structure that disables floating point exceptions while
+// it is alive, so that operations which generate NaNs and infinite values
+// intentionally will not trip an exception when debugging problematic
+// code that is generating NaNs and infinite values erroneously.
+#[derive(Clone, Debug, PartialEq, Eq)]
+pub(crate) struct DisableFloatingPointExceptionsFlags {
+    #[cfg(feature = "avoid-fe-exceptions")]
+    // We can't get a precise size for this, because it's of type
+    // `fenv_t`, which is a definition that doesn't exist in rust
+    // (not even in the libc crate, as of the time of writing.)
+    // But since the state is intended to be stored on the stack,
+    // 256 bytes should be more than enough.
+    original_flags: [u8; 256],
+}
+
+#[cfg(feature = "avoid-fe-exceptions")]
+extern "C" {
+    fn feholdexcept(env: *mut std::ffi::c_void);
+    fn fesetenv(env: *const std::ffi::c_void);
+}
+
+impl DisableFloatingPointExceptionsFlags {
+    #[cfg(not(feature = "avoid-fe-exceptions"))]
+    pub fn disable_floating_point_exceptions() -> Self {
+        Self { }
+    }
+
+    #[cfg(feature = "avoid-fe-exceptions")]
+    pub fn disable_floating_point_exceptions() -> Self {
+        unsafe {
+            let mut original_flags = [0; 256];
+            feholdexcept(original_flags.as_mut_ptr() as *mut _);
+            Self { original_flags }
+        }
+    }
+}
+
+#[cfg(feature = "avoid-fe-exceptions")]
+impl Drop for DisableFloatingPointExceptionsFlags {
+    fn drop(&mut self) {
+        unsafe {
+            fesetenv(self.original_flags.as_ptr() as *const _);
+        }
+    }
+}
+
 pub(crate) fn select_other<T: PartialEq>(pair: (T, T), elt: T) -> T {
     if pair.0 == elt {
         pair.1